Gradient coils in an MRI system serve for providing a magnetic field that is used for performing imaging measurements, and require a high voltage and high current that must be controlled. This voltage is typically in the order of 2000V and above. The required peak current is in the order of 600 A and above. To obtain detailed MRI images, the current must accurately and repeatedly follow a reference. The voltage and current for powering the gradient coils are provided by a power electronic circuit, comprising switching elements arranged in so called switching legs, together forming an H-bridge. These power electronic circuits with switching legs require a blanking time, i.e. a time wherein all switches of a leg are opened at the same time, which causes problems for accurately tracking of the reference at low output currents.
To generate the high output voltage and high output current a single H-bridge or stacked H-bridge multi-level power converter with output filter is typically used. For instance a single H-bridge with IGBT switches with anti-parallel diodes, or instead in principle any electronically controlled switch with a parallel diode can be used. The switches are controlled with a pulse width modulation scheme suitable for an H-bridge (for example unipolar or bipolar PWM). A stacked H-bridge with output filter consists of multiple H-bridges placed in series with an output filter. Where each H-bridge cell can be a basic H-bridge as described above, but can also be any functionally equivalent circuit composed by two switching legs of two series switches connected to a voltage source. Such a functionally equivalent circuit is a dV/dt filtered H-bridge. The output current of the converter flowing through a load will be referred to as the load current in the rest of this patent description.
The pulse width modulation scheme of the switches also includes a blanking time (dead time). This blanking time is a time where both top switches and bottom switches (in other words: all switches of one leg) are commanded to be off. This blanking time is added to prevent short circuiting the voltage source (shoot through) due to the turn-on and turn-off delays of the switches. Due to the required margins on the blanking time, there is a moment where both the top and bottom switch of a single switching leg are off, during this time the output voltage of the H-bridge is determined by the sign of the current in each switching leg, as this determines which parallel diodes will conduct. This current dependent output voltage has a negative effect on the converter output signal quality. The effect also generates a dead-band in the response of the output voltage/current on a changing control signal, specifically at low load currents. When using more H-bridge cells the effect of the blanking time becomes larger as each H-bridge cell requires a certain blanking time. To provide the required output quality (voltage/current waveform) to the gradient coils in an MRI application, the load current is typically controlled in closed-loop with a feedback circuit. The load current is measured and compared to a reference signal; based on the error the switching signals are changed to reduce the error. Because of the dead-band at low output currents, the output current is difficult to control, providing a poor output quality with a large error between the load current and the reference signal.
This problem is addressed in the prior art, in the international patent application WO2013/046099 for instance methods are proposed to compensate for the blanking time error by calculated compensation, while the international patent application WO2012/020363 suggests online adaptive model simulation. In the U.S. Pat. No. 6,535,402 the required compensation for the switching signals is determined based on the measured sign of the output current. The proposed methods provide model and measurement based compensation, which relies on measurements and knowledge of variable parasitic component parameters, limiting the performance of these methods.